In the past, attempts have been made to develop machining systems which have increased flexibility. Thus, for example, in U.S. Pat. No. 4,457,193 there is disclosed a machining apparatus which includes two coaxial spindles. One spindle is mounted on a stationary headstock, while the other spindle is carried on a movable headstock, which is designed to permit the transfer of workpieces between the spindles. Vertical tool turrets are carried by slides mounted on opposite sides of the axis of rotation of the spindles. The slides and hence the vertical tool turrets can move in a direction parallel to the axis of rotation of the spindle. The vertical tool turrets can also move in a direction perpendicular to the axis of rotation of the spindles.
The machining apparatus of the '193 patent has the capability of permitting two tools to simultaneously perform machining operations on a workpiece mounted on either one of the spindles. Also, the apparatus has the capability of permitting a workpiece mounted in one spindle to be machined by a tool mounted in one turret, while another workpiece mounted in the other spindle is machined by a tool mounted in the other turret. Although these capabilities give the apparatus some degree of flexibility, the flexibility of the apparatus is limited. For instance, because the vertical turrets are adapted to hold tools only, the machining apparatus does not have the capability of functioning as a way-type machine in which workpieces would be mounted in place of the turrets and the spindles, equipped with appropriate tools, would be moved toward the workpiece to perform a machining operation thereon. The inability to mount workpieces between the spindles also eliminates the capability of simultaneously machining both ends of a single workpiece. Because the vertical turrets are not adapted to receive workpieces, the workpieces must be mounted in the spindles and the tools must be mounted in the turrets. As a result of such a design constraint, the machining operations which can be performed by the apparatus are limited to those operations which can be performed by a non-rating tool on a rotating workpiece. Inasmuch as the vertical turrets are permanently mounted on the slides, the apparatus does not have the flexibility to replace the vertical turrets with horizontal turrets or with a jig milling attachment.